Nanofiltration Membrane Fouling and Control Caused by Residual Aluminum in Feed Water

It is reported that residual aluminum from coagulation pretreatment can increase membrane fouling in nanofiltration (NF) or reverse osmosis (RO) desalination systems. However, the membrane fouling mechanism of residual aluminum and the effective control measures are not very clear. In this study, the nanofiltration membrane fouling caused by poly-aluminum chloride (PACl), the inhibitory effect of amino trimethylene phosphonic acid (ATMP) on aluminum foulants, and calcium carbonate scale in the presence of residual aluminum were investigated by permeate experiments and scanning electron microscopy (SEM) observation of the fouled membranes. Besides, the effect of adding ultrafiltration before nanofiltration on reducing membrane fouling caused by residual aluminum was also investigated. The results showed that most of the residual aluminum in feed eventually formed insoluble particles and fine flocs, and accumulated gradually on the membrane surface, which was a typical colloid particle fouling. ATMP can reduce the membrane fouling caused by residual aluminum to some extent, and the permeate flux ratio (J/J(c)) increased from 0.83 to 0.89 when ATMP increased to the optimal dose of 6 mg.L-1. However, this means can still effectively eliminate carbonate scale, even in the presence of residual aluminum. Moreover, ultrafiltration pretreatment prior to nanofiltration was very effective to control membrane fouling, and the comprehensive cost was calculated to be only 0.006 $.m(-3). Therefore, the combined process of coagulation-ultrafiltration as a pretreatment of nanofiltration or reverse osmosis should be an ideal way to prevent membrane fouling caused by residual aluminum.